Subterranean formations that include coal seams can contain substantial quantities of adsorbed methane gas. Extracting this gas may help protect mining personnel from dangerous exposures to methane and may allow the producer to derive profit from sale of the gas as an energy source. Coal's unique structure allows it to store gas through adsorption onto its surface, which is covered with micro-pores. The high density of micro-pores yields 10 to 100 square meters of surface area per gram of coal, giving coal beds the capacity to store significant amounts of gas.
Generally, the closer wells are spaced, the greater gas recovery may be over the economic life of the wells. Ideally, wells are spaced to maximize gas liberation by minimizing the reservoir pressure in the coal seam across a large area. Coal seams are different from other hydrocarbon reservoirs in this respect—the reservoir pressure needs to be reduced to release the gas from coal. Because subterranean water often accompanies methane gas in coal seams, reservoir pressure can be reduced by removing this water while preventing localized water recharge. This reduction in water pressure can be achieved by spacing many wells in close proximity, with the actual distance between each well determined by the permeability of the coal seam, among other factors. The production of gas by one well will reduce the pressure in the reservoir and affect production by neighboring wells. This amount of well “interference” is determined by a number of factors, including, but not limited to, factors such as permeability, permeability anisotropy and well spacing. The reduced pressure resulting from this interference allows gas to desorb from the coal quicker, which improves the early economics of the field development. A more effective mechanism is to allow a higher pressure drop to be transmitted deeper into the formation. A fractured system is significantly more effective in accomplishing this than a radial flow system. Wells are spaced to yield maximum interference within four to six years. This spacing allows for maximum production within a feasible economic time frame. Furthermore, the less distance a gas or water molecule must travel to a well, the greater production will be within the economic time frame of the wells. Therefore, well spacing is a critical design element in any gas production system.
The fracturing of coal seams often requires very high pressures in comparison to other types of formations. In sandstone, for example, the fracture gradient may be 0.7 psi/ft or maybe 0.85 psi/ft or 0.9 psi/ft at the most. In coal seams, however, the pressure gradient may be 1.0, 1.2, 1.5 and even as high as 3.0. In a conventional rock formation, the fracture gradient normally represents the in situ stress, the minimum horizontal stress. In a coal seam, the fracture gradient represents stresses plus the difficulty to extend the fracture, and that difficulty can be greater than the magnitude of the stresses. There is a significant pressure drop due to tortuosity, which are twists and bends in the formation, which are pervasive in coal seams.
Prior solutions have been developed in an attempt to reduce the fracture gradient in coal. Typically, these solutions have been focused on optimizing the viscosity of the fracturing fluid. If water is used as the fracturing fluid, which has a viscosity of 1.0, a fracture gradient of 3.0 is generated. If a linear gel is used, the fracture gradient drops to 2.0-2.5. Foam yields a fracture gradient of 1.0-2.0 and a crosslinked gel yields a fracture gradient of 1.0-1.5. It turns out, contrary to logic, that the higher viscosity fluids yield lower stresses, and the lower viscosity fluids yield higher stresses. This is because a higher viscosity fluid gives a wider fracture—a single, dominant fracture, with few competing fractures. Although operators have had success with optimizing the fracturing fluids for coal seams, they are still confronted with fracture initiation difficulties associated with the perforations in cased wells, and fracture initiation and containment difficulties in open hole wells.
Another problem with fracturing coal seams is the creation of “near-well-bore stresses.” This occurs when the coal seam, which is naturally-fractured, is perforated and fractured and is particularly problematic in vertical wells because formation stresses vary with depth. The perforations, which are scattered along a varied depth of the well bore, create multiple and random entry points for the fracture fluid to flow into the formation. This random flowpath coupled with an already tortuous network of pathways within the coal seam formation results in a complex fracture, which is typically not aligned with the plane of maximum stress and lacks a single, dominate fracture. Thus, an inefficient and often sometimes ineffectual pathway for the gas to reach the well bore is created.